Front-projection-type image display devices are known wherein a projection lens is positioned on the same side of a screen as the audience, with the images being viewed by being reflected by the screen, and rear-projection-type image display devices are known wherein a screen is positioned between the audience and the projection lens, with images being formed by light that is scattered by the screen. In rear-projection-type image display devices, a construction is known wherein a cabinet encloses the light source and the projection lens, and the screen is arranged on the front surface of the cabinet.
FIG. 6 illustrates, in a vertical cross-section, the basic configuration of components of a rear-projection-type image display device according to the prior art. Images formed on a liquid crystal display panel 12 are magnified and projected onto a screen 16 by a projection lens 11. Nearly collimated light from a light source 15 is irradiated onto the pixels of the liquid crystal display panel 12, and thus a light beam carrying image information is projected onto the back side of the screen 16 via the projection lens 11. The audience is positioned at the front side of the screen 16 and thus it sees magnified images by viewing from the left side of FIG. 6. For simplicity of illustration, only one liquid crystal display panel 12 is shown. However, liquid crystal projectors for projecting full-color images are generally used. These liquid crystal projectors separate white light from the light source 15 into three primary colors, such as R, G, and B, using dichroic mirrors and a separate liquid crystal display panel modulates a given color component with image information for that color component. The three color-component beams are then synthesized by arranging dichroic reflecting surfaces on surfaces within a glass beam-combiner 13, as is well known in the art. The screen 16 is located at a fixed distance xe2x80x9cdxe2x80x9d from the liquid crystal display panel 12 that is nearest the screen.
In the past, in such a cabinet-type, image display device, it has been required to minimize the volume of the entire device. In response to such demand, projection devices as disclosed in Japanese examined application H8-201688 have been developed. To make the volume of the entire image display device smaller, it is vital to make the projection distance, d (the distance between the liquid crystal display panel nearest the screen and the screen) short. Therefore, the projection lens must be provided with a wide field of view. In this prior-art example, by folding the light path using mirrors, the image display device is made more compact. However, the mirrors result in an increase in the weight of the image display device. This is disadvantages, and thus it would be desirable to make such an image display device more compact without increasing the weight of the image display device. For this purpose, a projection lens having a wider image angle than in this prior-art examples desired.
In recent years, a display method called xe2x80x9cmulti-displayxe2x80x9d has frequently been used, wherein an image to be displayed on a large screen is broken into segments that are arranged in rows and columns, with each segment being projected by separate projection units that are positioned in an array from top-to-bottom and from side-to-side. In a rear-projection-type display device used for such a multi-display, projection lenses having their distortion aberration very well corrected are required in order that adjacent images overlap one another properly at shaded portions of a large screen. The less distortion in the adjacent images, the less these images need to overlap one another at shaded portions of a large screen. More specifically, the distortion should desirably be less than or equal to 0.3%. Also, lateral color has to be minimized to a very small amount or eliminated, since it degrades image quality.
A projection lens having a half-image angle that exceeds 40 degrees is termed a xe2x80x9cwide-anglexe2x80x9d projection lens. It is said that making a wide-angle projection lens requires exceedingly high skills because the lens is required to be telecentric on its reducing side in order that color artifacts are not introduced in modulating the light with the liquid crystal display panels. In addition, favorable correction of distortion and lateral color is required, and this is not easy. Generally, in making such a wide-angle lens, a diaphragm is employed between a front lens group and a back lens group. However, these lens groups extensively lack symmetry. This makes the correction of distortion and lateral color difficult. The requirement that the projection lens be telecentric on its reducing side makes the favorable correction of these aberrations even more difficult.
The present invention is a projection lens for a projection-type image display device which magnifies and projects image information that is input to a small display device, such as a liquid crystal display device or to a digital micro-mirror device (DMD). More particularly, the present invention is a wide-angle projection lens and a projection-type image display device that uses the projection lens.
The object of the invention is to provide a wide-angle projection lens having a shorter focal length and that is very well corrected for distortion aberration and lateral color so as to enable the wide-angle projection lens to be used in a rear-projection-type image display device that is more compact than image display devices currently available.